XML: Improve XML parsing error messages

This patch modifies error handling function for the XML parser provided
by libxml2.

Originaly only a line number and error message were logged. With this
new error handler function, the user is provided with a more complex
description of the parsing error.

Context of the error is printed in libXML2 style and filename of the
file, that caused the error is printed. Example of an parse error:

13:41:36.262: 16032: error : catchXMLError:706 :
/etc/libvirt/qemu/rh_bad.xml:58: Opening and ending tag mismatch: name
line 2 and domain
</domain>
---------^

Context of the error gives the user hints that may help to quickly
locate a corrupt xml file.

fixes BZs:
----------
Bug 708735 - [RFE] Show column and line on XML parsing error
https://bugzilla.redhat.com/show_bug.cgi?id=708735

Bug 726771 - libvirt does not specify problem file if persistent xml is
invalid
https://bugzilla.redhat.com/show_bug.cgi?id=726771

src/util/xml.c

@@ -633,28 +633,92 @@ virXPathNodeSet(const char *xpath,
  * catchXMLError:
  *
  * Called from SAX on parsing errors in the XML.
+ *
+ * This version is heavily based on xmlParserPrintFileContextInternal from libxml2.
  */
 static void
 catchXMLError(void *ctx, const char *msg ATTRIBUTE_UNUSED, ...)
 {
-    int domcode = VIR_FROM_XML;
     xmlParserCtxtPtr ctxt = (xmlParserCtxtPtr) ctx;
 
-    if (ctxt) {
-        if (ctxt->_private)
+    const xmlChar *cur, *base;
+    unsigned int n, col;	/* GCC warns if signed, because compared with sizeof() */
+    int domcode = VIR_FROM_XML;
+
+    virBuffer buf = VIR_BUFFER_INITIALIZER;
+    char *contextstr = NULL;
+    char *pointerstr = NULL;
+
+
+    /* conditions for error printing */
+    if (!ctxt ||
+        (virGetLastError() != NULL) ||
+        ctxt->input == NULL ||
+        ctxt->lastError.level != XML_ERR_FATAL ||
+        ctxt->lastError.message == NULL)
+        return;
+
+    if (ctxt->_private)
             domcode = ((struct virParserData *) ctxt->_private)->domcode;
 
-        if (virGetLastError() == NULL &&
-            ctxt->lastError.level == XML_ERR_FATAL &&
-            ctxt->lastError.message != NULL) {
-            virGenericReportError(domcode, VIR_ERR_XML_DETAIL,
-                                  _("at line %d: %s"),
-                                  ctxt->lastError.line,
-                                  ctxt->lastError.message);
-        }
+
+    cur = ctxt->input->cur;
+    base = ctxt->input->base;
+
+    /* skip backwards over any end-of-lines */
+    while ((cur > base) && ((*(cur) == '\n') || (*(cur) == '\r'))) {
+        cur--;
     }
-}
 
+    /* search backwards for beginning-of-line (to max buff size) */
+    while ((cur > base) && (*(cur) != '\n') && (*(cur) != '\r'))
+        cur--;
+    if ((*(cur) == '\n') || (*(cur) == '\r')) cur++;
+
+    /* calculate the error position in terms of the current position */
+    col = ctxt->input->cur - cur;
+
+    /* search forward for end-of-line (to max buff size) */
+    /* copy selected text to our buffer */
+    while ((*cur != 0) && (*(cur) != '\n') && (*(cur) != '\r')) {
+        virBufferAddChar(&buf, *cur++);
+    }
+
+    /* create blank line with problem pointer */
+    contextstr = virBufferContentAndReset(&buf);
+
+    /* (leave buffer space for pointer + line terminator) */
+    for  (n = 0; (n<col) && (contextstr[n] != 0); n++) {
+        if (contextstr[n] == '\t')
+            virBufferAddChar(&buf, '\t');
+        else
+            virBufferAddChar(&buf, '-');
+    }
+
+    virBufferAddChar(&buf, '^');
+
+    pointerstr = virBufferContentAndReset(&buf);
+
+    if (ctxt->lastError.file) {
+        virGenericReportError(domcode, VIR_ERR_XML_DETAIL,
+                              _("%s:%d: %s%s\n%s"),
+                              ctxt->lastError.file,
+                              ctxt->lastError.line,
+                              ctxt->lastError.message,
+                              contextstr,
+                              pointerstr);
+    } else {
+         virGenericReportError(domcode, VIR_ERR_XML_DETAIL,
+                              _("at line %d: %s%s\n%s"),
+                              ctxt->lastError.line,
+                              ctxt->lastError.message,
+                              contextstr,
+                              pointerstr);
+    }
+
+    VIR_FREE(contextstr);
+    VIR_FREE(pointerstr);
+}
 
 /**
  * virXMLParseHelper: